In fish aquaria used by hobbyists and professional fish breeders it is necessary to establish and maintain an environment that is not only habitable for the fish, but is as close as possible to the native environment of the fish. This is particularly important for breeding pairs, since even a small amount of environmental stress will prevent successful breeding for many species. While an aquarium soon after filling with fresh water usually has a suitable environment, nitrogenous wastes, such as left-over food, fecal and other aquatic life excretions, and other decaying plant/animal materials, are introduced over time into the aquarium. The nitrogenous wastes then break down through bacterial action into ammonia (NH.sub.3). Ammonia is very toxic, particularly to certain exotic fish species that are popular among aquarium owners. If the ammonia is not removed or transformed into a less toxic substance, the aquarium environment quickly degrades as the ammonia concentration climbs until it will no longer support fish life.
Some fish breeders solve the ammonia build-up problem by replacing a significant portion of the aquarium water, either continuously or at regular intervals. However, this is usually a laborious practice and impractical for most hobbyists and fish breeders. Accordingly, the aquarium water is usually circulated through filters or purifiers, or the like, to remove the ammonia and then recirculated back into the aquarium. The term "filter," as used herein and in the aquarium art, encompasses any device that removes undesirable substances from the water, whether dissolved chemical substances, or as solid particulates.
There are many types of filters available that operate on the basic principle of circulating water from the aquarium through a filter medium and then back into the aquarium. The medium in these filters is usually a medium for removing solid particles, e.g., floss, or a medium, such as activated carbon, or an ion exchange resin, that removes dissolved gasses or other dissolved substances.
While such simple filters are straight forward and relatively inexpensive, they are usually inadequate for large volume aquaria, or for sensitive fish species, s particularly breeding these fish species. For example, fresh water fish of the Cichlid family, commonly known as the discus fish, will breed only in very clean and quiescent water. In addition, these fish are sensitive to ammonia. Even a small concentration of ammonia in the aquarium water will stress the discus fish sufficiently to prevent breeding and will also render the fish more susceptible to disease.
Ammonia is naturally removed to a small degree from the aquarium by transformation into nitrites and then to nitrates by, for example, nitrosomonas and nitrobacters bacteria. However, unless artificially cultivated, there is an insufficient bacteria population to transform the i ammonia generated in the aquarium. An attempt to exploit this natural process is provided by the subgravel filter. In this filter water is sucked through the bottom gravel of the: aquarium by means of a plate under the gravel. However, it has been found that in order for sufficient ammonia to be removed by bacterial action, a large surface area must be provided as a habitat for the bacteria. With subgravel filters, there are usually dead spots through which water does not circulate, and even without the dead spots the surface area of the gravel is usually insufficient for an adequate bacteria population. Accordingly, a further means for removing the ammonia, or a large water replacement is also required to maintain the aquarium environment.
In conventional filters where water is siphoned into the filter box, passed through a medium and pumped back into the aquarium, there have been attempts to introduce a medium that provides a substrate for growing the bacteria that break down harmful substances. These include shapes of inert material, such as plastic, open pore plastic foams, and mineral substances pressed into shapes, such as diatomaceous earth. However, it has been found that merely adding a substrate for bacteria growing to a conventional filter does not always provide the bacterial population necessary to provide the required ammonia conversion, particularly where there is a high fish density.
One reason such is that the filters provide insufficient surface area for an adequate bacteria population. Even if the medium may have a high surface area, there is typically little environmental control to optimize the bacteria population. It has also been s found that optimal environments differ for the various bacteria that transform ammonia to nitrites, and nitrates. Some bacteria are "aerobic" and grow best in a highly aerated environment. Other bacteria varieties grow best in an "anaerobic," or less aerated environment or oxygen depleted environment. While many filters, both conventional and under-gravel filters, may support both types of bacteria, the environment of the filter medium is not well controlled, and it is impossible to establish or maintain the filter medium conditions for the optimal growth of both types of bacteria. In addition, the ideal balance between aerobic and anaerobic bacteria populations varies considerably, due to fish types and population density. Also, in a system with a high ammonia removal rate using a high anaerobic bacteria population, the anaerobic bacteria population can deplete dissolved oxygen in the water. Therefore, an anaerobic population which is higher than is necessary is undesirable. However, conventional and under-gravel filters typically have inadequate means to control the density or balance of bacteria population.
In an attempt to provide a large surface area, and provide an optimum bacteria habitat for both aerobic and anaerobic bacteria, so-called wet-dry filters have been developed. These filters comprise a "dry" section that includes a relatively large chamber filled with a medium or packing, e.g., plastic beads, or the like. Water is trickled down through to wet the surface of the medium of the dry section, which provides an aerobic habitat for bacteria.
The water then enters the "wet" section. The wet section is distinguished from the dry section in that the packing or medium of the wet section is submerged in water, or "wet," as opposed to being "dry" with water percolated over its surface. The packing of the wet section is typically an open cell foam, which provides interior spaces, which provide a sufficiently oxygen depleted environment for anaerobic bacterial growth.
The volume of the dry section in prior-art filters is quite large, e.g., typically about 2 cubic feet for a 110 gallon aquarium. This is equivalent to about 30 cubic inches per gallon aquarium capacity. The volume is required to provide sufficient medium surface area for bacterial growth for any medium that may be used, which is often spherical or shapes of plastic shapes. In addition, the dry section is typically placed above the wet section, typically separated therefrom by a perforated plate to support the dry section medium and allow water to pass into the wet section. Combined with the large dry section, this requires that the filter be relatively large and high. For this reason, the only suitable space for the filter is usually under the aquarium.
In a typical wet-dry filter system, water is drawn from the aquarium through siphon tubes into a prefilter box mounted on the back of the aquarium. The prefilter box provides one or more chambers, one usually containing a prefilter medium, e.g., floss, which removes large particles from the water.
From the prefilter, water flows down through a flexible tube into the filter assembly and onto a diffuser plate at the top of the dry section. The water then trickles through the dry section, through the perforated plate, and into the wet section. Typically the medium in the wet section is supported by a perforated plate spaced above the bottom of the filter to allow water leaving through the bottom of the wet section to pass laterally into a sump disposed to the side of the wet section. During normal operation of the filter, the wet section is maintained in a submerged state in a portion of the sump. From the sump the water is then pumped back into the .aquarium.
While the prior-art wet-dry filters are generally successful, they are subject to many operating difficulties. These difficulties derive mainly by the large volume of these filters. The size of the total assembly, the dry filter chamber and wet section/sump chamber, usually ranges from 2 to 3 feet long, and 1 to 2 feet high for filters of 100 to 400 gallon capacity. During operation of the filter, the wet section and the sump contain several gallons of water. If the circulation of this large volume of water is disrupted, either the aquarium or filter can overflow. For example, if there is a power failure and the pump stops, water will continue to flow: through the prefilter until the water level in the aquarium lowers to the level of the overflow baffle in the prefilter. However, if the water level is high in the aquarium the water flow may continue long enough to fill the sump, dry and wet sections, and overflow the filter. In addition, if the prefilter becomes clogged, even partially, and insufficient water flows from the prefilter, the pump can empty the sump and wet section into the aquarium. If the water level in the aquarium is high and them is insufficient excess capacity, the aquarium will then overflow. On the other hand if the water level in the aquarium is too low, the siphons into the prefilter will fail or the water in the prefilter will not rise sufficiently to flow over the baffle. The pump will then pump the filter dry, and if left on without water circulating through it, will burn out.
In summary, proper:operation of a wet-dry filter requires careful and frequent monitoring of the water level of the aquarium to prevent either a burned out pump or a flooded floor. This can become burdensome since the range between "too low" and "too high" for the aquarium level is quite narrow. The problem is aggravated by the large evaporating surface in the dry section.
The high evaporation rate not only requires a frequent monitoring of the water level, but also creates an undesirable humid condition in the confined space under the aquarium. Often the space under an aquarium is closed and used for auxiliary equipment, food storage, and the like. Thus, the placement of the filter under the aquarium creates a contained humid atmosphere, which contributes to corrosion of the equipment and deterioration of the food. This is a particular problem for salt-water aquaria, in which the filter also generates a salt-water mist.
The large volume of the wet-dry filter is required, not only because of the large volume of the dry section medium, but also to provide a large sump for the pump. The sump must be relatively large to accommodate fluctuations in the flow of water. Therefore, a significant volume of the filter is essentially empty space and makes no contribution to the purifying of the water. In addition, where a low ammonia removal rate is required, it is may be desirable to remove the wet section to prevent excessive oxygen depletion. In a conventional wet-dry filter, this can be provided by removing the medium. However, the volume formally occupied by the wet section medium, since it is flooded, cannot be used for additional dry medium, contributing to more wasted filter volume.
The sequential construction of prior-art wet-dry filters also does not permit flexibility in tailoring the flow of water through the wet and the dry sections. Basically, the same water flow goes through both the wet and the dry sections. It would be desirable to permit, for example, more water to flow through a dry section than through the wet section where the user requires more of the purification action from the dry-section. Alternately, it may be desirable to have a small or no flow through the dry section and a large flow through the wet section. Other variations may also be desirable.
Another problem with prior-art wet-dry filters is that they are usually quite noisy, aggravated by the fact that relatively high powered centrifugal pumps are required to pump a high volume of water from underneath the aquarium up to the aquarium top, against a water head typically from three to five feet. In addition, they are burdensome to clean. Cleaning requires that the filter be emptied, which often involves discarding and replacing the high water volume in the filter. Since the filters are usually underneath the aquarium, access into the dry and wet sections from the top is limited, which adds to the difficulty of replacing or cleaning the wet and dry section medium.
Mounting of a wet-dry filter in the back of the aquarium has been suggested. However, because of the large volume of the filter, the width of the aquarium must be significantly reduced or the aquarium/filter assembly becomes so wide it is unattractive and extends too far into the room. Mounting a wet-dry filter on the top of the aquarium would block most of the top access into the aquarium and would preclude the use of fluorescent light strips on the top. Accordingly, top-mounted filters are usually not of the wet-dry type but are small and of simple design, e.g., a simple trickle type or a type using filter canisters.
Another common problem, not only of prior-an wet-dry filters, but also of several filters where water is pumped by centrifugal pumps into and/or out of the aquarium, is that excessive turbulence is often created in the aquarium. Often a rough current from a single pump out-flow to the siphon inlets exists, which is undesirable for fish requiring quiescent water. In addition, prior-ann filters, because of the placement of the filter inlets and outlets, often provide a poor circulation pattern in the aquarium. Ideally, the water circulation should comprise slow circular currents extending through the volume of the aquarium. Instead, the circulation is often a strong current between the pump outlet and the siphons with significant regions that are more or less stagnant. This results in regional variations of concentration of dissolved substances, such as oxygen and pollutants, such as ammonia, and provides inefficient filtering of the water in the stagnant regions.
Another problem with prior-ann filters involves the heater. A means sometimes used for heating water in aquaria with wet-dry filter systems is to place a conventional submersible aquarium heater in the sump from which water is pumped back into the aquarium. In the event of a siphon failure, the sump will be pumped dry and the heater is likely to burn out. In addition, if the aquarium user forgets to turn off the heater when he empties the filter for cleaning or maintenance, the heater will burn out.